Mass flux of combustible solids at piloted ignition

Abstract

A study is presented on the effects of heat flux, oxygen concentration, and oxidizer flow velocity on the critical pyrolysate mass flux at piloted ignition for a polypropylene–glass fiber composite and PMMA. The experimental apparatus consists of a small-scale wind tunnel in which solid fuel samples are thermally irradiated in a boundary layer oxidizer flow. The experimental results show that the critical mass flux at ignition (fire point) increases with both heat flux and oxidizer flow velocity, but is insensitive to oxygen content over the range 18–27% by volume. A simplified theoretical model is developed to estimate the critical mass flux of a solid fuel at the (piloted) fire point. The model relates the critical mass flux for ignition to fuel properties that can be directly measured or calculated for arbitrary fuels (heat of combustion and stoichiometric oxidant to fuel mass ratio) as well as the environmental conditions. Predictions of the critical mass flux at the fire point are compared with experimental data, with generally good agreement.